A publicly known motorcycle body frame includes: a head pipe; a main frame extending downward obliquely and backward from it above an engine; a down frame extending downward obliquely and backward ahead of the engine; a lower frame extending longitudinally under the engine with its front end connected with the lower end of the down frame; and a center frame connecting the rear end of the lower frame and the rear end of the main frame vertically, where the down frame and the main frame are connected by a stiffener pipe.
It is also known that for the purpose of reinforcement, gussets formed by forging are welded to the joints of the down frame, formed from a wrought product made by extrusion molding, etc., with the stiffener pipe and with the lower frame (for example, see JP-A No. 2004-256073).